This invention relates to processes for selective capture and removal of purified carbon dioxide gas, the selective removal and recovery of sulfur dioxide and nitrogen oxides plus heavy and trace metals, such as, but not limited to mercury, selenium, cadmium, arsenic, germanium, uranium and beryllium from gaseous mixtures containing these constituents, and especially from flue gas streams having low concentrations. These gaseous mixtures include combustion flue gas or offgases produced from the burning of coal, oil, natural gas and other hydrocarbon fuels in power plants and industrial, agricultural and municipal furnaces and from similar emission sources.
It is particularly difficult to remove, by conventional means, the lower valence nitrogen oxides such as nitric oxide and trace metals including mercury vapor from these flue gas mixtures without prior preoxidation steps at the low concentrations at which they occur. It is also difficult to remove, by conventional means, the sulfur dioxide and carbon dioxide from these flue gas streams because of liquid solubility concerns in spite of their somewhat greater concentrations occurring separately or in the presence of other materials.
All of the above constituents are considered as air pollutants with a variety of adverse effects in the atmosphere and associated human environment. Sulfur dioxide and nitrogen oxides are air pollutants found at moderate concentrations in flue gas streams with well-established and regulated emission standards and ambient air quality standards by many countries. Mercury vapor and similar trace metallic constituents are air pollutants which are present at very low concentrations in flue gas streams which are potential problems with regard to human health, for which emission standards are only beginning to be developed. Carbon dioxide is a gaseous constituent found at higher concentrations in these flue gas streams, which is being increasingly linked as a contributor to the currently attributed global warming and climate change.
The present invention permits compliance with these regulatory mandates for air pollution control in a manner, which permits their recovery as commercially viable by-products in quantity and quality such that controlling and reusing these constituents may become profitable in the best case, and in the worst case as less costly than the conventional means of emissions control.
The present invention includes an aerodynamic reactor system, in which the heavy metal and particulate aerosols are removed and in which sulfur dioxide, carbon dioxide and nitrogen oxides are separated from the effluent gas stream following oxidation of mercury vapor, nitrogen oxides and organic compounds.
Various forms of wet scrubbers have been disclosed previously. For example: Raymond U.S. Pat. No. 467,264 shows an early method of purifying smoke by moistening it with steam and then separating the solid particles centrifugally. Jackson U.S. Pat. No. 723,531 discloses an apparatus for condensing smoke, fumes or gases by the use of a pair of water sprays and separating the particles by gravity or by filtration. British Pat. No. 925,711 discloses a liquid dust filter in which the dust-containing gas is accelerated through a narrowing passage or nozzle and then directed against a liquid stream. Gorman U.S. Pat. No. 3,894,851 utilizes water droplets condensed from steam to mix with a polluted gas. Thereafter, the wetted particulate is separated in a cyclone separator. Barnhart U.S. Pat. No. 3,812,656 discloses a fan driven venturi through which the dirt-laden air is drawn. Water is sprayed into the throat of the venturi to wet the dirt-laden air, and the dirt and water are separated by gravity while the air is exhausted through the fan. Another form of venturi scrubber is disclosed in Baum U.S. Pat. No. 3,898,308 which provides a series of adjustable water jets at the throat of the venturi for gas/liquid contact.
More recently, a series of patents have issued in which fine particulate is captured by encapsulating or entraining the particulate in small droplets and thereafter causing the droplets to grow in size until they can readily be separated from the gas in accordance with centrifugal or flow detachment separation principles. These patents include Ewan, et al. U.S. Pat. No. 3,852,408; Martin, Ewan et al. U.S. Pat. No. 3,852,409; Ewan, et al. U.S. Pat. No. 3,912,469; Ewan, et al. U.S. Pat. No. 4,141,701; Cason, Ewan et al. U.S. Pat. No. 4,272,499, and Frier, Bass and Ewan U.S. Pat. No. 6,447,574 B1. In the latter patent the flue gas flowing through the system is entirely subsonic and the system is not capable of separately removing the particulate pollutants and the acid gas pollutants, or to reclaim them separately and regenerate them as valuable by-products. Moreover, none of the above patents contains any technology or mechanism concerning the capture of carbon dioxide. Certain of the above patents also disclose the removal of acidic gases such as sulfur dioxide and nitrogen oxides by means of alkaline chemical reagents such as sodium carbonate, sodium hydroxide, calcium oxide, calcium hydroxide, magnesium oxide, ammonium hydroxide and potassium permanganate. The end products of these removal processes for all of the above patents are either relatively low value materials such as gypsum or ammonium based fertilizers or disposable material such as sludge or solid waste residues.
Another aspect of the present invention is the separation and recovery of sulfur, nitrogen, and carbon compounds from the effluent gases in a form such as potassium sulfate or potassium nitrate which are valuable as fertilizers. Carbon dioxide which is captured and recovered may be sold as an end product, to be used for the enhanced tertiary recovery of oil, or for enhanced vegetable crop growth, or to produce algae for BioDiesel fuel, or to be sequestered. Carbon dioxide may also be utilized as a feedstock to produce commercial end-products such as methanol, ethanol (which may be considered as a feedstock for making BioFuels and transportation fuels) and ethylene (with ethylene, or more specifically ethylene dichloride, being an intermediate for polyvinyl chloride plastics production).
A process for the removal and recovery of nitrogen and sulfur oxides from gaseous mixtures such as combustion gases from power plants is disclosed in Cooper U.S. Pat. Nos. 4,425,313 and 4,426,364. In these patents the nitrogen and sulfur oxides are removed separately or together and are converted to sulfates and nitrates which may be useful as fertilizers. However, these earlier patents do not disclose how to separate gases so as to prevent the heavy metals and very fine particulate matter (PM) in the flue gas: (1) from contaminating the recovered sulfates and nitrates to make them marketable as fertilizers in a purer form, or (2) from aerodynamically or chemically escaping capture and being released to the atmosphere. As noted above, heavy metals such as mercury and other trace metals and very fine particulate matter normally escape capture and are now recognized as air pollutants which can significantly affect the environment and human health.
The Powerspan Electro-Catalytic Oxidation Process described in U.S. Pat. Nos. 6,132,692 and 7,052,662 is an ammonium rather than potassium based process (utilizing both dry and wet removal processes), which produces, as its end products, the less valuable ammonium based fertilizers such as the potentially dangerous and explosive mixtures of ammonium nitrates and ammonium sulfates. Also, the Powerspan process employs catalytic oxidation of elemental mercury and nitric oxide which is less effective than the high temperature gas phase thermal oxidation employed in the present invention, and Powerspan will result in possible catalyst plugging or fouling.
The Airborne Sodium Bicarbonate Process described in U.S. Pat. Nos. 6,315,976; 6,375,824 and 6,334,990 is a sodium based conventional chemistry process in contrast to the present invention which is normally in its preferred embodiment a potassium based combined aerodynamics physics and chemistry process. As a result, the Airborne Process cannot recover carbon dioxide and requires the additional use of ammonium hydroxide as a make-up chemical, which results in the formation of ammonium sulfate and the potentially hazardous ammonium nitrate situation similar to the aforementioned Powerspan process.
Gansley U.S. Pat. No. 6,638,342 recently discloses another ammonium based conventional chemistry process for the removal of sulfur dioxide, nitrogen dioxide and trace or toxic metals. As the trace metals are removed simultaneously with the sulfur dioxide and the nitrogen dioxide, the ammonium based fertilizer (produced by the Gansley process), is inherently contaminated by the precipitated metallic salts, without offering any remedy for the aforesaid toxicity problem raised. The Gansley process merely attempts to partially remove the toxic metals; however, such effort appears to be very costly in energy consumption and inefficient, and could not produce commercially marketable and non-toxic by-products from the flue gas.
The Skyonic Sodium Carbonate Process described in the U.S. Patent Publication No. 20060185985 employs the electrolysis of sodium chloride to sodium hydroxide in order to capture carbon dioxide as sodium carbonate and bicarbonate. The carbon dioxide gas (which is intended to be a purified gas) is then liberated from the carbonate solution by hydrochloric acid produced from the hydrogen and chlorine. However, the process does not involve the removal of sulfur oxides, nitrogen oxides and trace metals, and produce only the low value sodium carbonate and bicarbonate as by-product chemicals. Since Skyonic makes no provision to oxidize the mercury or other trace metals upstream of its bubbling process, then the metallic solids would be accumulating in the liquid solution. In addition, the unreacted and untreated elemental mercury vapor can either escape aerodynamically in the body of the flue gas stream, or can also accumulate in the absorbing liquid and then be released together with the carbon dioxide gas to contaminate it. The Skyonic Process claims to be only able to remove 70 percent of the carbon dioxide emitted while consuming 30 percent of the power plant's overall energy output. Therefore, there are considerations of toxicity, impracticality and cost inefficiencies.
Cooper U.S. Pat. Nos. 6,969,486 and 6,676,912 relate to the photolytic oxidation of hydrogen peroxide to hydroxyl free radicals using ultra-violet light to oxidize nitric oxide, carbon monoxide and organic vapors respectively to nitrogen dioxide, carbon dioxide and water. These earlier patents do not state that elemental mercury vapor can be oxidized to mercuric salts separately and selectively upstream, and therefore to become separated from the nitrogen oxides scrubbing step downstream, thereby avoiding contamination of the potassium nitrates (similarly for the other trace metals). The solution is herein addressed, disclosed and produced by the present invention which solves the problem. Furthermore, in all of the afore-mentioned earlier patents, there is also no provision to recover and produce methanol or ethanol or ethylene, in contrast to the present invention.